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Provenancing silcrete in the Cape coastal zone: Implications for Middle Stone Age research in South Africa
Journal of Human Evolution 65 (2013) 682e688
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Provenancing silcrete in the Cape coastal zone: Implications for Middle Stone Age research in South Africa David J. Nash a, b, *, Sheila Coulson c, Sigrid Staurset c, Martin P. Smith a, J. Stewart Ullyott a a
School of Environment and Technology, University of Brighton, Lewes Road, Brighton BN2 4GJ, United Kingdom School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa c Institute of Archaeology, Conservation and History, University of Oslo, Blindernveien 11, 0315 Oslo, Norway b
a r t i c l e i n f o Article history: Received 17 May 2013 Accepted 7 July 2013 Available online 20 August 2013 Keywords: Africa Stone provenancing Geochemistry Canonical discriminant analysis
Introduction Silcrete is a term first used by Lamplugh (1902) to describe a highly resistant and well-cemented near-surface crust formed as a result of silica accumulating within and cementing a pre-existing soil, sediment, rock or weathered material (Nash and Ullyott, 2007). It is widespread in southern Africa, with some of the most extensive outcrops occurring around the Cape coast of South Africa (Fig. 1; Summerfield, 1983a; Roberts, 2003). In this region, silcretes demarcate ancient marine-planed surfaces, alluvial plains and river terraces (Roberts, 2003) (Fig. 2A), and display a range of features indicative of the role of pedogenic- and/or groundwater-related processes in their formation (Fig. 2BeD). The majority of outcrops are at a considerable elevation relative to present-day sea level. Silcrete is also a major archaeological raw material in the South African Stone Age. Due to its knapping properties (e.g., Brown et al., 2009; Villa et al., 2009b), it has been used to make a variety of tool types, and is one of the most widely utilized materials for artifact manufacture in the southwestern and southern Cape (Roberts, 2003). Silcrete is prevalent in lithic assemblages from the Middle Stone Age (MSA) (Table 1), particularly in those with Still Bay or Howiesons Poort components. Silcrete artifacts have been used to infer a range of behavioral traits during the MSA, including local
* Corresponding author. E-mail address: [email protected] (D.J. Nash). 0047-2484/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jhevol.2013.07.006
versus long-distance acquisition, increased mobility, exchange networks, technological complexity, knapping strategies, intentional heat treatment, stylistic change and even symbolic behavior (see Table 1). However, all of these inferences hinge upon first establishing the provenance of the silcrete raw material, whether as an indication of the distance of transport by early humans or as an initial step in the selection of materials for experimental replication studies. The potential for using silcrete in provenancing studies in South Africa has been hinted at over the last decade. Roberts (2003: 1), for example, suggests that “since the character of silcrete [in the Cape] varies geographically and since its occurrence is frequently localized” it might be used to infer Stone Age migration patterns. Ambrose (2006: 367), referring to geochemical data within Roberts’ memoir, states that regional differences in the bulk chemistry of Cape silcretes are “large enough to suggest that trace element and isotopic methods could be used to clearly differentiate sources.” However, in a more recent review, Ambrose (again citing Roberts, 2003) suggests that “chemical compositions of raw materials such as silcretes are similar over great distances” (Ambrose, 2012: 57). Recent work by Nash et al. (2013) indicates that both Roberts (2003) and Ambrose (2006) may have been correct. Silcretes in northern Botswana and Namibia have been shown to exhibit spatial differences in major and trace element geochemistry, controlled by subtle variations in the mineralogy of the Kalahari Group sediments within which they formed. These differences have been used to identify the transport of silcrete raw materials by early humans over distances of 220e295 km to Tsodilo Hills in northwest Botswana during the MSA. Given the wide variety of bedrock and sediment types within which Cape coastal silcretes are developed (Roberts, 2003), it would be expected that Cape silcretes would exhibit equivalent, if not more clearly discernible, chemical differences to those identified in the Kalahari. This article explores whether this is the case, through an analysis of the geochemistry of silcretes from selected sites across the Cape. We demonstrate that silcretes developed in association with different bedrock types do indeed have distinct chemical signatures and could therefore be used in provenancing
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Figure 1. Distribution of silcrete in the Cape coastal zone (after Roberts, 2003; Minichillo, 2006; Will et al., 2013), together with the locations of silcrete sampling sites and the prominent Middle Stone Age sites listed in Table 1. Blombos Cave (BBC), Die Kelders 1 (DK), Diepkloof (DRS), Hoedjiespunt 1 (HDP), Hollow Rock Shelter (HRS), Howiesons Poort (HP), Klasies River e Main Site (KRM), Klein Kliphuis (KK), Montagu Cave (MC), Nelson Bay Cave (NBC), Pinnacle Point (PP), and Ysterfontein (YF).
studies. We conclude with a consideration of the implications of our results for MSA archaeology in South Africa. Materials and methods As a first stage in our investigation, silcrete profiles from across the Cape coastal zone were described and sampled (Fig. 1). The goals of the sampling strategy were to collect representative materials from well-documented areas of outcrop and to sample silcretes developed in association with a range of rock types. Sampling was not intended to provide a comprehensive inventory of all areas of exposure; this is a goal for future research. Initial sampling sites were targeted from a review of previous studies
(notably Frankel and Kent, 1938; Bosazza, 1939; Mountain, 1946; Frankel, 1952; Summerfield, 1978, 1981, 1983b, 1983c; 1984) with further localities identified during the course of fieldwork. Details of the 12 sampling sites are given in Table 2, with locations indicated in Fig. 1. The majority of the sampling sites were in contemporary quarries or road cuttings. This allowed access to full silcrete profiles, including any underlying exposed weathered bedrock. At each site, the outcrop was surveyed and logged, with samples taken at regular vertical intervals from a representative profile. Normally at least three samples were taken from each outcrop to ensure that any within-profile variability was incorporated into subsequent geochemical analyses. An exception was the Lutzville area where thick exposures of silcrete are rare. Where exposed, a sample of the
Figure 2. Silcrete in the Cape coastal zone. (A) Natural silcrete outcrop near Albertinia, with a silcrete-capped ancient land surface visible in the background; (B) quarry face, revealing deeply weathered Dwyka Group bedrock (base), passing upwards into an iron-rich weathered zone and capped by a 2.25 m thick pedogenic silcrete (profile SA96/4, east of Grahamstown); (C) uppermost part of a quarry face showing deeply weathered Dwyka Group bedrock overlain by a 2.30 m thick pedogenic silcrete horizon showing distinctive columnar structures (profile SA96/5, Enniskillen Farm, north of Grahamstown); (D) quarry face, revealing c. 13 m of deeply weathered Bokkeveld Group sediments capped by a massive 4.00 m silcrete, which preserves original sedimentary structures (profile SA96/11, Rooikop, north of Albertinia). Full details of profile locations are given in Table 2.
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Table 1 Silcrete in Middle Stone Age layers from selected prominent sites in the Cape coastal zone (see Fig. 1 for approximate locations). Site
Percentage silcrete in assemblage
Blombos Cave (BBC)
30e74% (Henshilwood et al., 2001; Henshilwood, 2005)
Die Kelders 1 (DK)
0e61% (Grine et al., 1991) 0e5% (Avery et al., 1997) 0e34% (Thackeray, 2000)
Diepkloof (DRS)
>50% (Rigaud et al., 2006) 4e66% (Porraz et al., 2013)
Hoedjiespunt 1 (HDP)
6% (Will et al., 2013)
Hollow Rock Shelter (HRS) Howiesons Poort (HP)
5e19% (Evans, 1994) 4e19% (Högberg and Larsson, 2011) 83e86% from 1927/1928 collection 60% from 1965 excavations (Deacon, 1995b) Presence noted especially in Howiesons Poort levels (percentage incorporated into the 33% non-local materials) (Wurz, 2000, 2010)
Klasies River e Main Site (KRM)
Klein Kliphuis (KK)
Montague Cave (MC) Nelson Bay Cave (NBC) Pinnacle Point (PP)
Ysterfontein (YF)
10e60% (Mackay, 2006) 73% in Howiesons Poort layers (Mackay, 2011a) 5e14% (Volman, 1981) 0e6% (Volman, 1981) Cave 13: 4% (Marean et al., 2004) Cave 13B: 3e13% (Thompson et al., 2010) 51% (Halkett et al., 2003) 47% (Wurz, 2012)
Major inferences from silcrete assemblage Local acquisition of raw material (Villa et al., 2005, 2009b). Replication of points required pressure flaking (Mourre et al., 2010) e silcrete for experimental study was acquired from nearby source (Mourre et al., 2010). Impact fractures inferred as evidence of hunting - based on comparison with experimental replications (Villa et al., 2009a,b). Non-local acquisition of raw material (Grine et al., 1991). Local acquisition of raw material (Thackeray, 2000). High percentage of silcrete a potential chronological marker (Grine et al., 1991; Avery et al., 1997; Thackeray, 2000). Raw material acquisition from surrounding and exotic sources (Porraz et al., 2008, 2013) e variability of silcrete sources determined using hand specimen characteristics (Porraz et al., 2008). Long-distance transport of raw material (Rigaud et al., 2006; Mackay, 2008, 2011b; Texier et al., 2010; Charrié-Duhaut et al., 2013; Igreja and Porraz, 2013). Hafting and adhesives found on Howiesons Poort artifacts made from silcrete acquired from both local and exotic sources (Charrié-Duhaut et al., 2013). Stylistic differences to nearby Klein Kliphuis (Mackay, 2011b). Long-distance transport of raw material e minimum 10 to 30 km (Will et al., 2013). Diepkloof Rock Shelter Silcrete Database used to source silcrete and determine transport distance to site (Will et al., 2013). Import of raw material (Evans, 1994) and finished points (Högberg and Larsson, 2011). Frequent use of silcrete characterizes the Howiesons Poort period (Deacon, 1995b).
Acquisition of raw material not from the immediate vicinity (Butzer, 1978; Singer and Wymer, 1982; Thackeray, 1989; Wurz, 1999; Lombard, 2005; Villa et al., 2010). Cultural preference for silcrete (Deacon and Geleijnse, 1988; Deacon, 1989). Non-local silcrete sources should be identifiable (Ambrose, 2006). Tools part of system of exchange (Deacon, 1995a). Symbolic behavior (Wurz, 1999, 2010). Local acquisition of raw material (Mackay, 2006). Stylistic differences to nearby Diepkloof (Mackay, 2011b). Varied amounts and quality of silcrete during Howiesons Poort (Mackay, 2011a). Higher percentage of silcrete use in the Late MSA-2 in comparison to the Howiesons Poort (Volman, 1981). Acquisition of raw material potentially dependent on changes in sea and dune levels (Volman, 1981). Raw material import from 10 to 15 km away (Marean et al., 2004; Minichillo, 2006). Intentional heat treatment for knapping e silcrete for experimental study selected using hand specimen characteristics (Brown et al., 2009, 2012). Silcrete the primary raw material (Halkett et al., 2003; Klein et al., 2004; Avery et al., 2008). Local acquisition of raw material (Wurz, 2012).
The percentage of silcrete shown for each assemblage is adjusted to the nearest integer.
Of the 64 samples, 50 comprised >85% SiO2 and can be considered as silcrete sensu stricto (Summerfield, 1983a; Nash and Ullyott, 2007). Data from these samples only were used in subsequent statistical analyses. The geochemistry of 11 replicate silcrete samples (selected at random from the 50 silcrete samples) was also determined. These included seven samples from the RiversdaleAlbertinia area (developed upon weathered Bokkeveld Group
underlying weathered bedrock was also collected. A GPS reading was taken at each site. To establish their geochemical signature, 64 silcrete and weathered bedrock samples were analyzed using a combination of ICP-MS, ICP-AES and other standard methods. The analytical procedure was identical to that described in Nash et al. (2013) and is summarized in Appendix A (Supplementary Online Material, SOM).
Table 2 Details of sample sites (see Fig. 1 for approximate locations). Site number SA96/1 SA96/2 SA96/4 SA96/5 SA96/6 SA96/7 SA96/8 SA96/9 SA96/10 SA96/11 SA96/12 SA96/13
Location West of Lutzville West of Lutzville East of Grahamstown Enniskillin Farm, North of Grahamstown Makanna’s Kop, Grahamstown West of Grahamstown East of Grahamstown Herbertsdale North of Albertinia Rooikop Farm, North of Albertinia Northwest of Albertinia Heidelberg
Latitude/Longitude 31 31 33 33 33 33 33 33 34 34 34 34
340 300 180 150 170 190 180 590 070 080 090 050
0000 1700 1100 5300 5400 2000 1100 4300 3000 4900 1500 4900
S, S, S, S, S, S, S, S, S, S, S, S,
018 018 026 026 026 026 026 021 021 021 021 020
240 080 360 300 340 290 340 470 360 320 260 440
0000 4700 4600 0300 0500 5800 5400 5600 5800 2100 5500 0300
E E E E E E E E E E E E
Associated bedrock
No. samples
Gifberg Group (Aties Formation) Gifberg Group (Aties Formation) Dwyka Group Dwyka Group Dwyka Group Lake Mentz Subgroup Dwyka Group Bokkeveld Group Bokkeveld Group Bokkeveld Group Bokkeveld Group Bokkeveld Group
1 2 4 3 3 5 3 4 9 9 3 4
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lithologies) and four from around Grahamstown (weathered Dwyka Group deposits). Full results and detection limits for all elements are shown in Appendix B (SOM). A total of 60 geochemical variables per sample (including major, trace, rare earth and volatile elements) were then analyzed by canonical discriminant analysis using SPSS. Two sets of analyses were undertaken: (i) with silcrete samples entered into the discriminant analysis grouped by associated bedrock type; (ii) with samples grouped by site. Details of the protocol used during statistical analyses are given in Appendix A. Geochemical discrimination of Cape silcretes The results of the canonical discriminant analysis of silcrete samples according to associated bedrock type are shown in Fig. 3A. The first two statistical functions arising from the analysis, which together explain 95.9% of the variance (Table 3), are plotted. The results indicate that silcretes developed in association with different lithologies can be discriminated on the basis of their chemistry with a high degree of statistical significance (Table 4). Silcretes developed upon weathered Bokkeveld Group and Dwyka Group deposits cluster in two discrete areas of the discriminant function plot. This indicates that the two sets of silcretes are chemically distinct. The restricted spread of samples around their respective group centroids further suggests a degree of chemical homogeneity within each cluster. Although represented by smaller numbers of samples, silcretes developed upon Gifberg Group and Lake Mentz Subgroup (part of the Witteberg Group) sediments also form distinct clusters. Fig. 3A additionally displays the results for the 11 replicate silcrete samples (shown as open symbols). Geochemical data for these samples were entered independently into the statistical analysis (see Appendix A) in order to (i) assess the chemical homogeneity of silcretes developed in association with specific rock types, and (ii) ascertain the robustness of the statistical discrimination method. If silcretes were chemically homogenous and the statistical method robust, then it would be expected that the data
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Table 3 Eigenvalues for the first three functions resulting from the canonical discriminant analysis of Cape silcretes by associated bedrock type (Functions 1 and 2 shown in Fig. 3A). Function 1 2 3
Eigenvalue
% of variance
Cumulative %
Canonical correlation
23.305 5.298 1.237
78.1 17.8 4.1
78.1 95.9 100.0
0.979 0.917 0.744
points for the replicate samples would plot close to the group centroids of their respective bedrock clusters. This is the case for replicates from the Riversdale-Albertinia area, which all fall close to the centre of the Bokkeveld Group cluster in Fig. 3A. The scatter is greater for the replicates associated with Dwyka Group sediments. This suggests that silcretes developed upon Dwyka Group sediments are less chemically homogenous than those formed in Bokkeveld Group deposits. This may be because some silcretes in the Grahamstown area were developed partly within younger (and more mineralogically variable) sediments overlying weathered Dwyka Group bedrock (see Roberts, 2003). Nonetheless, based on the squared Mahalanobis distance to the nearest group centroid, all four Dwyka Group silcrete replicates are classified with the other Dwyka Group materials. The chemical components that differentiate silcretes developed upon the different bedrock types can be identified by examining the standardized canonical discriminant function coefficients arising from the statistical analysis (Table 5). The elements exhibiting the highest coefficients (positive or negative) make the greatest contribution to each respective statistical function. The main contributors to Function 1 in Fig. 3A are the major oxide TiO2, the transition metal Nb and the rare earth element (REE) Ce. For Function 2, Nb and the REEs Er, Eu and Gd are most significant. Analysis of chondrite normalized REE abundance patterns (calculated using data from Wakita et al. (1971)) indicates that all samples are strongly light REE enriched, with flat heavy REE profiles and marked negative Eu anomalies. The range in absolute concentration
Figure 3. (A) Results of canonical discriminant analysis of geochemical data from the Cape coastal silcretes by associated bedrock type, showing the first two functions arising from the statistical analysis. The silcrete raw material samples developed upon weathered Bokkeveld Group, Dwyka Group, Gifberg Group and Lake Mentz Subgroup lithologies form four discrete groupings. The positions of 11 duplicate silcrete samples (open symbols) are shown in relation to these groups. (B) and (C): Results of canonical discriminant analysis of geochemical data for individual silcrete sampling sites in the Grahamstown (B) and Riversdale-Albertinia (C) areas. The plots indicate that samples from each site are chemically distinct.
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Table 4 Wilks’ Lambda and Chi-square statistics for the first three functions resulting from the canonical discriminant analysis of Cape silcretes by associated bedrock type (Functions 1 and 2 shown in Fig. 3A). Test of function(s)
Wilks’ Lambda
Chi-square
Degrees of freedom
Significance
1 through 3 2 through 3 3
0.003 0.071 0.447
180.921 82.010 24.946
96 62 30
0.000 0.045 0.770
between and within formations is probably a reflection of varying degrees of dilution of the host rock REE signature by silica precipitated during silcrete formation. As a further test of the methodology, geochemical data for silcretes from specific sampling sites in the Grahamstown and Riversdale-Albertinia areas were analyzed to assess whether (i) the silcretes at each site were chemically homogenous, and (ii) discriminant analysis could be used to distinguish chemical differences between sites. Fig. 3B and C show that samples from distinct sites form tight clusters around their respective group centroids, i.e., they are effectively chemically homogenous, such that surface samples are likely to be representative of whole profiles. Furthermore, the clusters for each site are distinct and do not overlap. For the Grahamstown area, Functions 1 (influenced by levels of Fe2O3, TiO2, MgO and SrO) and 2 (CaO, Fe2O3 and Al2O3) together explain 97.8% of the variance. For the Riversdale-Albertinia area, Functions 1 (SiO2, TiO2, Ce, Cs, Eu and Rb) and 2 (CaO, C, Cs and Rb) explain 76.0% of the variance, with a third function (Cs, Dy, Eu,
Table 5 Standardized canonical discriminant function coefficients, for all variables passing the minimum tolerance level (0.001), arising from the analysis of Cape silcretes by associated bedrock type. Element
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Cr2O3 TiO2 MnO P2O5 SrO BaO C S Ba Ce Cr Cs Dy Er Eu Ga Gd Hf La Nb Sr As Se Co Ni
Function 1
2
3
1.886 0.515 1.848 0.467 0.646 0.471 1.331 0.837 11.479 0.074 0.928 0.417 3.539 0.022 0.264 2.715 7.330 0.149 1.746 3.059 2.375 0.504 2.789 4.017 2.857 1.401 15.815 1.612 0.923 0.699 0.600 0.382
0.837 0.666 0.265 0.665 0.789 0.747 0.655 0.210 0.484 1.312 1.127 0.263 1.415 0.082 0.844 1.207 0.461 0.643 0.775 1.307 5.806 13.923 1.389 13.435 0.848 2.420 3.289 0.416 0.250 0.564 0.427 0.062
0.034 1.282 0.038 1.311 0.676 0.000 0.903 0.449 1.413 0.501 0.273 0.211 5.196 1.241 0.168 5.003 2.453 0.545 1.685 4.374 2.998 2.809 1.310 3.628 0.196 4.349 2.062 0.054 0.730 0.154 0.202 0.088
Functions 1 and 2 are shown in Fig. 3A.
Rb) explaining an additional 17.6%. Statistical data for the analyses by site are given in Tables S1eS6 in Appendix A. Archaeological implications The results presented in Fig. 3 demonstrate that, like the Kalahari silcretes analyzed by Nash et al. (2013), silcretes from the Cape coastal zone can be used in archaeological provenancing investigations. The results also suggest that, with sufficiently detailed sampling, it should be possible to chemically characterize individual silcrete quarry sites in the Cape. Most promisingly, our findings show that silcretes can be used as a provenancing medium regardless of the mechanism by which they developed; the sampled Cape silcretes formed largely by pedogenic processes whilst Kalahari silcretes are the product of non-pedogenic silicification (Roberts, 2003; Nash and Ullyott, 2007; Nash, 2012). The methodology presented here (and in Nash et al., 2013) has the potential to open up new areas of Stone Age research in the Cape coastal zone, but has particular resonance for the MSA. The identification of silcrete sources utilized during this period would facilitate in-depth understanding of a range of prehistoric human behaviors (see Table 1). Our approach has perhaps the greatest implications for future research into patterns of mobility and raw material acquisition. Inferences about local versus long-distance raw material acquisition frequently assume exploitation of the closest raw material sources. However, as Nash et al. (2013) have shown, this assumption may not always be correct. The disadvantage of our provenancing approach is that it is destructive. This is a minor issue when analyzing raw material samples, as in this study, but becomes more critical when attempting to match irreplaceable artifacts against their potential source. However, under our approach, it is possible to use fragments of knapping debris as small as 5 g when establishing the geochemical signature of archaeological material, so that the overall impact on collections is minimized (Nash et al., 2013). The nature of our methodology thus makes archaeological sample selection a key issue. Use of a technology-based analysis, such as the chaîne opératoire, in this process helps to ensure representivity. It also allows the provenanced materials to be linked directly to prehistoric behavior patterns at each site. At this stage, we would not recommend the use of our approach on artifacts that have been heat-treated. Recent work by Schmidt et al. (2013) has shown that heat-treatment of silcrete leads to the loss of chemically-bound silanole water from silica minerals and the hardening of the material through the formation of new SieOeSi bonds. These processes have the potential to affect the SiO2 content of samples. As of yet, the impact of heating upon the concentrations of other major and trace elements within silcrete (e.g., as a result of the dehydration of clay minerals) is not known. With these caveats, there are a number of productive avenues for future research using our methodology. A geochemical provenancing study focused on a single MSA site on the Cape coast (Fig. 1) could, for example, quantify distances of raw material transport to that site and permit the identification of any changes in resource acquisition patterns over time. This may also provide an answer to the question of what exactly is ‘local’ or ‘long-distance’ transport. A more extensive survey, analyzing silcretes from multiple sites, would permit patterning of wider resource acquisition strategies, such as early exchange networks. This could provide substantial insights, particularly into the complexity of human behavior visible in archaeological material from the Still Bay and Howiesons Poort periods. Finally, geochemical provenancing would add greater rigor to those investigations which have used only hand specimen characteristics (such as grain size, degree of cementation, level of
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translucence, type of cortex, color and presence of rinds, patches or specks in other colors) to identify silcrete source localities and/or select silcrete for experimental replication studies (see Table 1). As Nash et al. (2013) have demonstrated, this practice is unreliable for silcrete and should always be supported by geochemical data. Any future provenancing studies in the Cape coastal zone will need to bear in mind that additional areas of silcrete could have been exposed during late Pleistocene sea level lowstands. Rogers (1980), for example, reports silcrete horizons at depths of up to 50 m below present-day sea level from boreholes in the Noordhoek valley southwest of Cape Town, and it is possible that these outcrop in the slopes of the now-flooded coastal plain. Such additional sources are likely to be limited in extent, as the majority of Cape silcretes appear to be late Cretaceous to Neogene in age and are associated with now-uplifted land surfaces (Roberts, 2003). However, given the coincidence of some sea level lowstands (e.g., during Marine Isotope Stage 4; Fisher et al., 2010) with periods of peak silcrete use at coastal MSA sites (e.g., Pinnacle Point; Brown et al., 2009, 2012), the possibility of submerged raw material sources should not be overlooked. Acknowledgments Funding for fieldwork and geochemical analyses was provided by the University of Brighton. DJN would like to thank Joanna Bullard for assistance in the field during the collection of silcrete samples. Peter Lyons assisted with sample preparation. We also extend our thanks to the two anonymous reviewers whose helpful insights greatly improved the manuscript. Geochemical analyses were undertaken by ALS Minerals, Sevilla, Spain, and are certified under quality control certificate SV12045103 issued on 26 March 2012. Appendix. Supplementary data Supplementary methodological information (Appendix A) and data (Appendix B) related to this article can be found at http://dx. doi.org/10.1016/j.jhevol.2013.07.006. References Ambrose, S.H., 2006. Howiesons Poort lithic raw material procurement patterns and the evolution of modern human behavior: a response to Minichillo (2006). J. Hum. Evol. 50, 365e369. Ambrose, S.H., 2012. Obsidian dating and source exploitation studies in Africa: implications for the evolution of human behavior. In: Liritzis, I., Stevenson, C. (Eds.), Obsidian and Ancient Manufactured Glasses. University of New Mexico Press, Albuquerque, pp. 56e72. Avery, G., Cruz-Uribe, K., Goldberg, P., Grine, F.E., Klein, R.G., Lenardi, M.J., Marean, C.W., Rink, W.J., Schwarz, H.P., Thackeray, A.I., Wilson, M.L., 1997. The 1992e1993 excavations at the Die Kelders Middle and Later Stone Age Cave Site, South Africa. J. Field Archaeol. 24, 263e291. Avery, G., Halkett, D., Orton, J., Steele, T., Tusenius, M., Klein, R., 2008. The Ysterfontein 1 Middle Stone Age rock shelter and the evolution of coastal foraging. Goodwin Ser. Curr. Themes Middle Stone Age Res. 10, 66e89. Bosazza, V.L., 1939. The Silcrete and Clays of the Riversdale-Mossel Bay Area, vol. 32. Fulcrum, Johannesburg, pp. 17e29. Brown, K.S., Marean, C.W., Herries, A.I.R., Jacobs, Z., Tribolo, C., Braun, D., Roberts, D.L., Meyer, M.A., Bernatchez, J., 2009. Fire as an engineering tool of early modern humans. Science 325, 859e862. Brown, K.S., Marean, C.W., Jacobs, Z., Schoville, B.J., Oestmo, S., Fisher, E.C., Bernatchez, J., Karkanas, P., Matthews, T., 2012. An early and enduring advanced technology originating 71,000 years ago in South Africa. Nature 491, 590e594. Butzer, K.W., 1978. Sediment stratigraphy of the Middle Stone Age sequence at Klasies River Mouth, Tsitsikama Coast, South Africa. S. Afr. Archaeol. Bull. 33, 141e151. Charrié-Duhaut, A., Porraz, G., Cartwright, C.R., Igreja, M., Connan, J., Poggenpoel, C., Texier, P.-J., 2013. First molecular identification of a hafting adhesive in the Late Howiesons Poort at Diepkloof Rock Shelter (Western Cape, South Africa). J. Archaeol. Sci. 40, 3506e3518. Deacon, H.J., 1989. Late Pleistocene palaeoecology and archaeology in the Southern Cape, South Africa. In: Mellars, P., Stringer, C. (Eds.), The Human Revolution:
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Blombos Cave, Southern Cape, South Africa: preliminary report on the 1992e1999 excavations of the Middle Stone Age levels. J. Archaeol. Sci. 28, 421e448. Högberg, A., Larsson, L., 2011. Lithic technology and behavioral modernity: new results from the Still Bay site, Hollow Rock Shelter, Western Cape Province, South Africa. J. Hum. Evol. 61, 133e155. Igreja, M., Porraz, G., 2013. Functional insights into the innovative Early Howiesons Poort technology at Diepkloof Rock Shelter (Western Cape, South Africa). J. Archaeol. Sci. 40, 3475e3491. Klein, R.G., Avery, G., Cruz-Uribe, K., Halkett, D., Parkington, J.E., Steele, T., Volman, T.P., Yates, R., 2004. The Ysterfontein 1 Middle Stone Age site, South Africa, and early human exploitation of coastal resources. Proc. Natl. Acad. Sci. 101, 5708e5715. Lamplugh, G.W., 1902. Calcrete. Geol. Mag. 9, 75. Lombard, M., 2005. The Howiesons Poort of South Africa: what we know, what we think we know, what we need to know. S. Afr. Humanit. 17, 33e55. Mackay, A., 2006. A characterization of the MSA Stone artefact assemblage from the 1984 excavations at Klein Kliphuis, Western Cape. S. Afr. Archaeol. Bull. 61,181e188. Mackay, A., 2008. A method for estimating edge length from flake dimensions: use and implications for technological change in the southern African MSA. J. Archaeol. Sci. 35, 614e622. Mackay, A., 2011a. Nature and significance of the Howiesons Poort to postHowiesons Poort transition at Klein Kliphuis rockshelter, South Africa. J. Archaeol. Sci. 38, 1430e1440. Mackay, A., 2011b. Potentially stylistic differences between backed artefacts from two nearby sites occupied w60,000 years before present in South Africa. J. Anthropol. Archaeol. 30, 235e245. Marean, C.W., Nilssen, P.J., Brown, K., Jerardino, A., Stynder, D., 2004. Paleoanthropological investigations of Middle Stone Age sites at Pinnacle Point, Mossel Bay (South Africa): archaeology and hominid remains from the 2000 Field Season. Paleoanthropology 2, 14e83. Minichillo, T., 2006. Raw material use and behavioral modernity: Howiesons Poort lithic foraging strategies. J. Hum. Evol. 50, 359e364. Mountain, E.D., 1946. The Geology of the Area East of Grahamstown. Department of Mines, Pretoria. Mourre, V., Villa, P., Henshilwood, C.S., 2010. Early use of pressure flaking on lithic artifacts at Blombos Cave, South Africa. Science 330, 659e662. Nash, D.J., 2012. Duricrusts. In: Holmes, P.J., Meadows, M.E. (Eds.), Southern African Geomorphology: Recent Trends and New Directions. Sun Media, Bloemfontein, pp. 191e229. Nash, D.J., Ullyott, J.S., 2007. Silcrete. In: Nash, D.J., McLaren, S.J. (Eds.), Geochemical Sediments and Landscapes. Wiley-Blackwell, Oxford, pp. 95e143. Nash, D.J., Coulson, S., Staurset, S., Ullyott, J.S., Babutsi, M., Hopkinson, L., Smith, M.P., 2013. Provenancing of silcrete raw materials indicates longdistance transport to Tsodilo Hills, Botswana, during the Middle Stone Age. J. Hum. Evol. 64, 280e288. Porraz, G., Texier, P.-J., Rigaud, J.-P., Parkington, J., Poggenpoel, C., Roberts, D.L., 2008. Preliminary characterization of a Middle Stone Age lithic assemblage preceding the ‘Classic’ Howieson’s Poort Complex at Diepkloof Rock Shelter, Western Cape Province, South Africa. Goodwin Series. Curr. Themes Middle Stone Age Res. 10, 105e121. Porraz, G., Texier, P.-J., Archer, W., Piboule, M., Rigaud, J.-P., Tribolo, C., 2013. Technological successions in the Middle Stone Age sequence of Diepkloof Rock Shelter, Western Cape, South Africa. J. Archaeol. Sci. 40, 3376e3400. Rigaud, J.-P., Texier, P.-J., Parkington, J., Poggenpoel, C., 2006. Le mobilier Stillbay et Howiesons Poort de l’abri Diepkloof. La chronologie du Middle Stone Age sudafricain et ses implications. C.R. Palevol. 5, 839e849.
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Roberts, D.L., 2003. Age, Genesis and Significance of South African Coastal Belt Silcretes. Memoir 95, Council for Geoscience, Pretoria. Rogers, J., 1980. First Report on the Cenozoic Sediments between Cape Town and Elands Bay. Report 136. Geological Survey of South Africa, Pretoria. Schmidt, P., Porraz, G., Slodczyk, A., Bellot-Gurlet, L., Archer, W., Miller, C.E., 2013. Heat treatment in the South African Middle Stone Age: temperature induced transformations of silcrete and their technological implications. J. Archaeol. Sci. 40, 3519e3531. Singer, R., Wymer, J., 1982. The Middle Stone Age at Klasies River Mouth in South Africa. University of Chicago Press, Chicago. Summerfield, M.A., 1978. The Nature and Origin of Silcrete with Particular Reference to Southern Africa. Ph.D. Dissertation, University of Oxford. Summerfield, M.A., 1981. The Nature and Occurrence of Silcrete, Southern Cape Province, South Africa. School of Geography Research Paper 28. University of Oxford. Summerfield, M.A., 1983a. Silcrete. In: Goudie, A.S., Pye, K. (Eds.), Chemical Sediments and Geomorphology. Academic Press, London, pp. 59e91. Summerfield, M.A., 1983b. Petrography and diagenesis of silcrete from the Kalahari Basin and Cape Coastal Zone, southern Africa. J. Sediment. Petrol. 53, 895e909. Summerfield, M.A., 1983c. Geochemistry of weathering profile silcretes, southern Cape Province, South Africa. In: Wilson, R.C.L. (Ed.), Residual Deposits: Surface Related Weathering Processes and Materials. Geological Society, London, pp. 167e178. Special Publication No. 11. Summerfield, M.A., 1984. Isovolumetric weathering and silcrete formation, southern Cape Province, South Africa. Earth Surf. Proc. Landf. 9, 135e141. Texier, P.-J., Porraz, G., Parkington, J., Rigaud, J.-P., Poggenpoel, C., Miller, C., Tribolo, C., Cartwright, C., Coudenneau, A., Klein, R., Steele, T., Verna, C., 2010. A Howiesons Poort tradition of engraving ostrich eggshell containers dated to 60,000 years ago at Diepkloof Rock Shelter, South Africa. Proc. Natl. Acad. Sci. 107, 6180e6185. Thackeray, A.I., 1989. Changing fashions in the Middle Stone Age: the stone artefact sequence from Klasies River main site, South Africa. Afr. Archaeol. Rev. 7, 33e57.
Thackeray, A.I., 2000. Middle Stone Age artefacts from the 1993 and 1995 excavations of Die Kelders Cave 1, South Africa. J. Hum. Evol. 38, 147e168. Thompson, E., Williams, H.M., Minichillo, T., 2010. Middle and late Pleistocene Middle Stone Age lithic technology from Pinnacle Point 13B (Mossel Bay, Western Cape Province, South Africa). J. Hum. Evol. 59, 358e377. Villa, P., Delagnes, A., Wadley, L., 2005. A late Middle Stone Age artifact assemblage from Sibudu (KwaZulu-Natal): comparisons with the European Middle Paleolithic. J. Archaeol. Sci. 32, 399e422. Villa, P., Boscato, P., Ranaldo, F., Ronchitelli, A., 2009a. Stone tools for the hunt: points with impact scars from a Middle Paleolithic site in southern Italy. J. Archaeol. Sci. 36, 850e859. Villa, P., Soressi, M., Henshilwood, C.S., Mourre, V., 2009b. The Still Bay points of Blombos Cave (South Africa). J. Archaeol. Sci. 36, 441e460. Villa, P., Soriano, S., Teyssandier, N., Wurz, S., 2010. The Howiesons Poort and MSA III at Klasies River main site, Cave 1A. J. Archaeol. Sci. 37, 630e655. Volman, T.P., 1981. The Middle Stone Age in the Southern Cape. Ph.D. Dissertation, University of Chicago. Wakita, H., Rey, P., Schmitt, R.A., 1971. Abundances of the 14 rare-earth elements and 12 other trace elements in Apollo 12 samples: five igneous and one breccia rocks and four soils. Geochim. Cosmochim. Ac. Suppl. 2, 1319e1329. Will, M., Parkington, J.E., Kandel, A.W., Conard, N.J., 2013. Coastal adaptations and the Middle Stone Age lithic assemblages from Hoedjiespunt 1 in the Western Cape, South Africa. J. Hum. Evol. 64, 518e537. Wurz, S., 1999. The Howiesons Poort backed artefacts from Klasies River: an argument for symbolic behaviour. S. Afr. Archaeol. Bull. 54, 38e50. Wurz, S., 2000. The Middle Stone Age at Klasies River, South Africa. Ph.D. Dissertation, University of Stellenbosch. Wurz, S., 2010. Middle Stone Age stone tools from Klasies River Main Site and symbolic cognition. In: Nowell, A., Davidson, I. (Eds.), The Cutting Edge: Stone Tools and the Evolution of Cognition. Colorado University Press, Boulder, pp. 135e158. Wurz, S., 2012. The significance of MIS 5 shell middens on the Cape coast: a lithic perspective from Klasies River and Ysterfontein 1. Quatern. Int. 270, 61e69.
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Provenancing silcrete in the Cape coastal zone: Implications for Middle Stone Age research in South Africa David J. Nash a, b, *, Sheila Coulson c, Sigrid Staurset c, Martin P. Smith a, J. Stewart Ullyott a a
School of Environment and Technology, University of Brighton, Lewes Road, Brighton BN2 4GJ, United Kingdom School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa c Institute of Archaeology, Conservation and History, University of Oslo, Blindernveien 11, 0315 Oslo, Norway b
a r t i c l e i n f o Article history: Received 17 May 2013 Accepted 7 July 2013 Available online 20 August 2013 Keywords: Africa Stone provenancing Geochemistry Canonical discriminant analysis
Introduction Silcrete is a term first used by Lamplugh (1902) to describe a highly resistant and well-cemented near-surface crust formed as a result of silica accumulating within and cementing a pre-existing soil, sediment, rock or weathered material (Nash and Ullyott, 2007). It is widespread in southern Africa, with some of the most extensive outcrops occurring around the Cape coast of South Africa (Fig. 1; Summerfield, 1983a; Roberts, 2003). In this region, silcretes demarcate ancient marine-planed surfaces, alluvial plains and river terraces (Roberts, 2003) (Fig. 2A), and display a range of features indicative of the role of pedogenic- and/or groundwater-related processes in their formation (Fig. 2BeD). The majority of outcrops are at a considerable elevation relative to present-day sea level. Silcrete is also a major archaeological raw material in the South African Stone Age. Due to its knapping properties (e.g., Brown et al., 2009; Villa et al., 2009b), it has been used to make a variety of tool types, and is one of the most widely utilized materials for artifact manufacture in the southwestern and southern Cape (Roberts, 2003). Silcrete is prevalent in lithic assemblages from the Middle Stone Age (MSA) (Table 1), particularly in those with Still Bay or Howiesons Poort components. Silcrete artifacts have been used to infer a range of behavioral traits during the MSA, including local
* Corresponding author. E-mail address: [email protected] (D.J. Nash). 0047-2484/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jhevol.2013.07.006
versus long-distance acquisition, increased mobility, exchange networks, technological complexity, knapping strategies, intentional heat treatment, stylistic change and even symbolic behavior (see Table 1). However, all of these inferences hinge upon first establishing the provenance of the silcrete raw material, whether as an indication of the distance of transport by early humans or as an initial step in the selection of materials for experimental replication studies. The potential for using silcrete in provenancing studies in South Africa has been hinted at over the last decade. Roberts (2003: 1), for example, suggests that “since the character of silcrete [in the Cape] varies geographically and since its occurrence is frequently localized” it might be used to infer Stone Age migration patterns. Ambrose (2006: 367), referring to geochemical data within Roberts’ memoir, states that regional differences in the bulk chemistry of Cape silcretes are “large enough to suggest that trace element and isotopic methods could be used to clearly differentiate sources.” However, in a more recent review, Ambrose (again citing Roberts, 2003) suggests that “chemical compositions of raw materials such as silcretes are similar over great distances” (Ambrose, 2012: 57). Recent work by Nash et al. (2013) indicates that both Roberts (2003) and Ambrose (2006) may have been correct. Silcretes in northern Botswana and Namibia have been shown to exhibit spatial differences in major and trace element geochemistry, controlled by subtle variations in the mineralogy of the Kalahari Group sediments within which they formed. These differences have been used to identify the transport of silcrete raw materials by early humans over distances of 220e295 km to Tsodilo Hills in northwest Botswana during the MSA. Given the wide variety of bedrock and sediment types within which Cape coastal silcretes are developed (Roberts, 2003), it would be expected that Cape silcretes would exhibit equivalent, if not more clearly discernible, chemical differences to those identified in the Kalahari. This article explores whether this is the case, through an analysis of the geochemistry of silcretes from selected sites across the Cape. We demonstrate that silcretes developed in association with different bedrock types do indeed have distinct chemical signatures and could therefore be used in provenancing
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Figure 1. Distribution of silcrete in the Cape coastal zone (after Roberts, 2003; Minichillo, 2006; Will et al., 2013), together with the locations of silcrete sampling sites and the prominent Middle Stone Age sites listed in Table 1. Blombos Cave (BBC), Die Kelders 1 (DK), Diepkloof (DRS), Hoedjiespunt 1 (HDP), Hollow Rock Shelter (HRS), Howiesons Poort (HP), Klasies River e Main Site (KRM), Klein Kliphuis (KK), Montagu Cave (MC), Nelson Bay Cave (NBC), Pinnacle Point (PP), and Ysterfontein (YF).
studies. We conclude with a consideration of the implications of our results for MSA archaeology in South Africa. Materials and methods As a first stage in our investigation, silcrete profiles from across the Cape coastal zone were described and sampled (Fig. 1). The goals of the sampling strategy were to collect representative materials from well-documented areas of outcrop and to sample silcretes developed in association with a range of rock types. Sampling was not intended to provide a comprehensive inventory of all areas of exposure; this is a goal for future research. Initial sampling sites were targeted from a review of previous studies
(notably Frankel and Kent, 1938; Bosazza, 1939; Mountain, 1946; Frankel, 1952; Summerfield, 1978, 1981, 1983b, 1983c; 1984) with further localities identified during the course of fieldwork. Details of the 12 sampling sites are given in Table 2, with locations indicated in Fig. 1. The majority of the sampling sites were in contemporary quarries or road cuttings. This allowed access to full silcrete profiles, including any underlying exposed weathered bedrock. At each site, the outcrop was surveyed and logged, with samples taken at regular vertical intervals from a representative profile. Normally at least three samples were taken from each outcrop to ensure that any within-profile variability was incorporated into subsequent geochemical analyses. An exception was the Lutzville area where thick exposures of silcrete are rare. Where exposed, a sample of the
Figure 2. Silcrete in the Cape coastal zone. (A) Natural silcrete outcrop near Albertinia, with a silcrete-capped ancient land surface visible in the background; (B) quarry face, revealing deeply weathered Dwyka Group bedrock (base), passing upwards into an iron-rich weathered zone and capped by a 2.25 m thick pedogenic silcrete (profile SA96/4, east of Grahamstown); (C) uppermost part of a quarry face showing deeply weathered Dwyka Group bedrock overlain by a 2.30 m thick pedogenic silcrete horizon showing distinctive columnar structures (profile SA96/5, Enniskillen Farm, north of Grahamstown); (D) quarry face, revealing c. 13 m of deeply weathered Bokkeveld Group sediments capped by a massive 4.00 m silcrete, which preserves original sedimentary structures (profile SA96/11, Rooikop, north of Albertinia). Full details of profile locations are given in Table 2.
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Table 1 Silcrete in Middle Stone Age layers from selected prominent sites in the Cape coastal zone (see Fig. 1 for approximate locations). Site
Percentage silcrete in assemblage
Blombos Cave (BBC)
30e74% (Henshilwood et al., 2001; Henshilwood, 2005)
Die Kelders 1 (DK)
0e61% (Grine et al., 1991) 0e5% (Avery et al., 1997) 0e34% (Thackeray, 2000)
Diepkloof (DRS)
>50% (Rigaud et al., 2006) 4e66% (Porraz et al., 2013)
Hoedjiespunt 1 (HDP)
6% (Will et al., 2013)
Hollow Rock Shelter (HRS) Howiesons Poort (HP)
5e19% (Evans, 1994) 4e19% (Högberg and Larsson, 2011) 83e86% from 1927/1928 collection 60% from 1965 excavations (Deacon, 1995b) Presence noted especially in Howiesons Poort levels (percentage incorporated into the 33% non-local materials) (Wurz, 2000, 2010)
Klasies River e Main Site (KRM)
Klein Kliphuis (KK)
Montague Cave (MC) Nelson Bay Cave (NBC) Pinnacle Point (PP)
Ysterfontein (YF)
10e60% (Mackay, 2006) 73% in Howiesons Poort layers (Mackay, 2011a) 5e14% (Volman, 1981) 0e6% (Volman, 1981) Cave 13: 4% (Marean et al., 2004) Cave 13B: 3e13% (Thompson et al., 2010) 51% (Halkett et al., 2003) 47% (Wurz, 2012)
Major inferences from silcrete assemblage Local acquisition of raw material (Villa et al., 2005, 2009b). Replication of points required pressure flaking (Mourre et al., 2010) e silcrete for experimental study was acquired from nearby source (Mourre et al., 2010). Impact fractures inferred as evidence of hunting - based on comparison with experimental replications (Villa et al., 2009a,b). Non-local acquisition of raw material (Grine et al., 1991). Local acquisition of raw material (Thackeray, 2000). High percentage of silcrete a potential chronological marker (Grine et al., 1991; Avery et al., 1997; Thackeray, 2000). Raw material acquisition from surrounding and exotic sources (Porraz et al., 2008, 2013) e variability of silcrete sources determined using hand specimen characteristics (Porraz et al., 2008). Long-distance transport of raw material (Rigaud et al., 2006; Mackay, 2008, 2011b; Texier et al., 2010; Charrié-Duhaut et al., 2013; Igreja and Porraz, 2013). Hafting and adhesives found on Howiesons Poort artifacts made from silcrete acquired from both local and exotic sources (Charrié-Duhaut et al., 2013). Stylistic differences to nearby Klein Kliphuis (Mackay, 2011b). Long-distance transport of raw material e minimum 10 to 30 km (Will et al., 2013). Diepkloof Rock Shelter Silcrete Database used to source silcrete and determine transport distance to site (Will et al., 2013). Import of raw material (Evans, 1994) and finished points (Högberg and Larsson, 2011). Frequent use of silcrete characterizes the Howiesons Poort period (Deacon, 1995b).
Acquisition of raw material not from the immediate vicinity (Butzer, 1978; Singer and Wymer, 1982; Thackeray, 1989; Wurz, 1999; Lombard, 2005; Villa et al., 2010). Cultural preference for silcrete (Deacon and Geleijnse, 1988; Deacon, 1989). Non-local silcrete sources should be identifiable (Ambrose, 2006). Tools part of system of exchange (Deacon, 1995a). Symbolic behavior (Wurz, 1999, 2010). Local acquisition of raw material (Mackay, 2006). Stylistic differences to nearby Diepkloof (Mackay, 2011b). Varied amounts and quality of silcrete during Howiesons Poort (Mackay, 2011a). Higher percentage of silcrete use in the Late MSA-2 in comparison to the Howiesons Poort (Volman, 1981). Acquisition of raw material potentially dependent on changes in sea and dune levels (Volman, 1981). Raw material import from 10 to 15 km away (Marean et al., 2004; Minichillo, 2006). Intentional heat treatment for knapping e silcrete for experimental study selected using hand specimen characteristics (Brown et al., 2009, 2012). Silcrete the primary raw material (Halkett et al., 2003; Klein et al., 2004; Avery et al., 2008). Local acquisition of raw material (Wurz, 2012).
The percentage of silcrete shown for each assemblage is adjusted to the nearest integer.
Of the 64 samples, 50 comprised >85% SiO2 and can be considered as silcrete sensu stricto (Summerfield, 1983a; Nash and Ullyott, 2007). Data from these samples only were used in subsequent statistical analyses. The geochemistry of 11 replicate silcrete samples (selected at random from the 50 silcrete samples) was also determined. These included seven samples from the RiversdaleAlbertinia area (developed upon weathered Bokkeveld Group
underlying weathered bedrock was also collected. A GPS reading was taken at each site. To establish their geochemical signature, 64 silcrete and weathered bedrock samples were analyzed using a combination of ICP-MS, ICP-AES and other standard methods. The analytical procedure was identical to that described in Nash et al. (2013) and is summarized in Appendix A (Supplementary Online Material, SOM).
Table 2 Details of sample sites (see Fig. 1 for approximate locations). Site number SA96/1 SA96/2 SA96/4 SA96/5 SA96/6 SA96/7 SA96/8 SA96/9 SA96/10 SA96/11 SA96/12 SA96/13
Location West of Lutzville West of Lutzville East of Grahamstown Enniskillin Farm, North of Grahamstown Makanna’s Kop, Grahamstown West of Grahamstown East of Grahamstown Herbertsdale North of Albertinia Rooikop Farm, North of Albertinia Northwest of Albertinia Heidelberg
Latitude/Longitude 31 31 33 33 33 33 33 33 34 34 34 34
340 300 180 150 170 190 180 590 070 080 090 050
0000 1700 1100 5300 5400 2000 1100 4300 3000 4900 1500 4900
S, S, S, S, S, S, S, S, S, S, S, S,
018 018 026 026 026 026 026 021 021 021 021 020
240 080 360 300 340 290 340 470 360 320 260 440
0000 4700 4600 0300 0500 5800 5400 5600 5800 2100 5500 0300
E E E E E E E E E E E E
Associated bedrock
No. samples
Gifberg Group (Aties Formation) Gifberg Group (Aties Formation) Dwyka Group Dwyka Group Dwyka Group Lake Mentz Subgroup Dwyka Group Bokkeveld Group Bokkeveld Group Bokkeveld Group Bokkeveld Group Bokkeveld Group
1 2 4 3 3 5 3 4 9 9 3 4
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lithologies) and four from around Grahamstown (weathered Dwyka Group deposits). Full results and detection limits for all elements are shown in Appendix B (SOM). A total of 60 geochemical variables per sample (including major, trace, rare earth and volatile elements) were then analyzed by canonical discriminant analysis using SPSS. Two sets of analyses were undertaken: (i) with silcrete samples entered into the discriminant analysis grouped by associated bedrock type; (ii) with samples grouped by site. Details of the protocol used during statistical analyses are given in Appendix A. Geochemical discrimination of Cape silcretes The results of the canonical discriminant analysis of silcrete samples according to associated bedrock type are shown in Fig. 3A. The first two statistical functions arising from the analysis, which together explain 95.9% of the variance (Table 3), are plotted. The results indicate that silcretes developed in association with different lithologies can be discriminated on the basis of their chemistry with a high degree of statistical significance (Table 4). Silcretes developed upon weathered Bokkeveld Group and Dwyka Group deposits cluster in two discrete areas of the discriminant function plot. This indicates that the two sets of silcretes are chemically distinct. The restricted spread of samples around their respective group centroids further suggests a degree of chemical homogeneity within each cluster. Although represented by smaller numbers of samples, silcretes developed upon Gifberg Group and Lake Mentz Subgroup (part of the Witteberg Group) sediments also form distinct clusters. Fig. 3A additionally displays the results for the 11 replicate silcrete samples (shown as open symbols). Geochemical data for these samples were entered independently into the statistical analysis (see Appendix A) in order to (i) assess the chemical homogeneity of silcretes developed in association with specific rock types, and (ii) ascertain the robustness of the statistical discrimination method. If silcretes were chemically homogenous and the statistical method robust, then it would be expected that the data
685
Table 3 Eigenvalues for the first three functions resulting from the canonical discriminant analysis of Cape silcretes by associated bedrock type (Functions 1 and 2 shown in Fig. 3A). Function 1 2 3
Eigenvalue
% of variance
Cumulative %
Canonical correlation
23.305 5.298 1.237
78.1 17.8 4.1
78.1 95.9 100.0
0.979 0.917 0.744
points for the replicate samples would plot close to the group centroids of their respective bedrock clusters. This is the case for replicates from the Riversdale-Albertinia area, which all fall close to the centre of the Bokkeveld Group cluster in Fig. 3A. The scatter is greater for the replicates associated with Dwyka Group sediments. This suggests that silcretes developed upon Dwyka Group sediments are less chemically homogenous than those formed in Bokkeveld Group deposits. This may be because some silcretes in the Grahamstown area were developed partly within younger (and more mineralogically variable) sediments overlying weathered Dwyka Group bedrock (see Roberts, 2003). Nonetheless, based on the squared Mahalanobis distance to the nearest group centroid, all four Dwyka Group silcrete replicates are classified with the other Dwyka Group materials. The chemical components that differentiate silcretes developed upon the different bedrock types can be identified by examining the standardized canonical discriminant function coefficients arising from the statistical analysis (Table 5). The elements exhibiting the highest coefficients (positive or negative) make the greatest contribution to each respective statistical function. The main contributors to Function 1 in Fig. 3A are the major oxide TiO2, the transition metal Nb and the rare earth element (REE) Ce. For Function 2, Nb and the REEs Er, Eu and Gd are most significant. Analysis of chondrite normalized REE abundance patterns (calculated using data from Wakita et al. (1971)) indicates that all samples are strongly light REE enriched, with flat heavy REE profiles and marked negative Eu anomalies. The range in absolute concentration
Figure 3. (A) Results of canonical discriminant analysis of geochemical data from the Cape coastal silcretes by associated bedrock type, showing the first two functions arising from the statistical analysis. The silcrete raw material samples developed upon weathered Bokkeveld Group, Dwyka Group, Gifberg Group and Lake Mentz Subgroup lithologies form four discrete groupings. The positions of 11 duplicate silcrete samples (open symbols) are shown in relation to these groups. (B) and (C): Results of canonical discriminant analysis of geochemical data for individual silcrete sampling sites in the Grahamstown (B) and Riversdale-Albertinia (C) areas. The plots indicate that samples from each site are chemically distinct.
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Table 4 Wilks’ Lambda and Chi-square statistics for the first three functions resulting from the canonical discriminant analysis of Cape silcretes by associated bedrock type (Functions 1 and 2 shown in Fig. 3A). Test of function(s)
Wilks’ Lambda
Chi-square
Degrees of freedom
Significance
1 through 3 2 through 3 3
0.003 0.071 0.447
180.921 82.010 24.946
96 62 30
0.000 0.045 0.770
between and within formations is probably a reflection of varying degrees of dilution of the host rock REE signature by silica precipitated during silcrete formation. As a further test of the methodology, geochemical data for silcretes from specific sampling sites in the Grahamstown and Riversdale-Albertinia areas were analyzed to assess whether (i) the silcretes at each site were chemically homogenous, and (ii) discriminant analysis could be used to distinguish chemical differences between sites. Fig. 3B and C show that samples from distinct sites form tight clusters around their respective group centroids, i.e., they are effectively chemically homogenous, such that surface samples are likely to be representative of whole profiles. Furthermore, the clusters for each site are distinct and do not overlap. For the Grahamstown area, Functions 1 (influenced by levels of Fe2O3, TiO2, MgO and SrO) and 2 (CaO, Fe2O3 and Al2O3) together explain 97.8% of the variance. For the Riversdale-Albertinia area, Functions 1 (SiO2, TiO2, Ce, Cs, Eu and Rb) and 2 (CaO, C, Cs and Rb) explain 76.0% of the variance, with a third function (Cs, Dy, Eu,
Table 5 Standardized canonical discriminant function coefficients, for all variables passing the minimum tolerance level (0.001), arising from the analysis of Cape silcretes by associated bedrock type. Element
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Cr2O3 TiO2 MnO P2O5 SrO BaO C S Ba Ce Cr Cs Dy Er Eu Ga Gd Hf La Nb Sr As Se Co Ni
Function 1
2
3
1.886 0.515 1.848 0.467 0.646 0.471 1.331 0.837 11.479 0.074 0.928 0.417 3.539 0.022 0.264 2.715 7.330 0.149 1.746 3.059 2.375 0.504 2.789 4.017 2.857 1.401 15.815 1.612 0.923 0.699 0.600 0.382
0.837 0.666 0.265 0.665 0.789 0.747 0.655 0.210 0.484 1.312 1.127 0.263 1.415 0.082 0.844 1.207 0.461 0.643 0.775 1.307 5.806 13.923 1.389 13.435 0.848 2.420 3.289 0.416 0.250 0.564 0.427 0.062
0.034 1.282 0.038 1.311 0.676 0.000 0.903 0.449 1.413 0.501 0.273 0.211 5.196 1.241 0.168 5.003 2.453 0.545 1.685 4.374 2.998 2.809 1.310 3.628 0.196 4.349 2.062 0.054 0.730 0.154 0.202 0.088
Functions 1 and 2 are shown in Fig. 3A.
Rb) explaining an additional 17.6%. Statistical data for the analyses by site are given in Tables S1eS6 in Appendix A. Archaeological implications The results presented in Fig. 3 demonstrate that, like the Kalahari silcretes analyzed by Nash et al. (2013), silcretes from the Cape coastal zone can be used in archaeological provenancing investigations. The results also suggest that, with sufficiently detailed sampling, it should be possible to chemically characterize individual silcrete quarry sites in the Cape. Most promisingly, our findings show that silcretes can be used as a provenancing medium regardless of the mechanism by which they developed; the sampled Cape silcretes formed largely by pedogenic processes whilst Kalahari silcretes are the product of non-pedogenic silicification (Roberts, 2003; Nash and Ullyott, 2007; Nash, 2012). The methodology presented here (and in Nash et al., 2013) has the potential to open up new areas of Stone Age research in the Cape coastal zone, but has particular resonance for the MSA. The identification of silcrete sources utilized during this period would facilitate in-depth understanding of a range of prehistoric human behaviors (see Table 1). Our approach has perhaps the greatest implications for future research into patterns of mobility and raw material acquisition. Inferences about local versus long-distance raw material acquisition frequently assume exploitation of the closest raw material sources. However, as Nash et al. (2013) have shown, this assumption may not always be correct. The disadvantage of our provenancing approach is that it is destructive. This is a minor issue when analyzing raw material samples, as in this study, but becomes more critical when attempting to match irreplaceable artifacts against their potential source. However, under our approach, it is possible to use fragments of knapping debris as small as 5 g when establishing the geochemical signature of archaeological material, so that the overall impact on collections is minimized (Nash et al., 2013). The nature of our methodology thus makes archaeological sample selection a key issue. Use of a technology-based analysis, such as the chaîne opératoire, in this process helps to ensure representivity. It also allows the provenanced materials to be linked directly to prehistoric behavior patterns at each site. At this stage, we would not recommend the use of our approach on artifacts that have been heat-treated. Recent work by Schmidt et al. (2013) has shown that heat-treatment of silcrete leads to the loss of chemically-bound silanole water from silica minerals and the hardening of the material through the formation of new SieOeSi bonds. These processes have the potential to affect the SiO2 content of samples. As of yet, the impact of heating upon the concentrations of other major and trace elements within silcrete (e.g., as a result of the dehydration of clay minerals) is not known. With these caveats, there are a number of productive avenues for future research using our methodology. A geochemical provenancing study focused on a single MSA site on the Cape coast (Fig. 1) could, for example, quantify distances of raw material transport to that site and permit the identification of any changes in resource acquisition patterns over time. This may also provide an answer to the question of what exactly is ‘local’ or ‘long-distance’ transport. A more extensive survey, analyzing silcretes from multiple sites, would permit patterning of wider resource acquisition strategies, such as early exchange networks. This could provide substantial insights, particularly into the complexity of human behavior visible in archaeological material from the Still Bay and Howiesons Poort periods. Finally, geochemical provenancing would add greater rigor to those investigations which have used only hand specimen characteristics (such as grain size, degree of cementation, level of
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translucence, type of cortex, color and presence of rinds, patches or specks in other colors) to identify silcrete source localities and/or select silcrete for experimental replication studies (see Table 1). As Nash et al. (2013) have demonstrated, this practice is unreliable for silcrete and should always be supported by geochemical data. Any future provenancing studies in the Cape coastal zone will need to bear in mind that additional areas of silcrete could have been exposed during late Pleistocene sea level lowstands. Rogers (1980), for example, reports silcrete horizons at depths of up to 50 m below present-day sea level from boreholes in the Noordhoek valley southwest of Cape Town, and it is possible that these outcrop in the slopes of the now-flooded coastal plain. Such additional sources are likely to be limited in extent, as the majority of Cape silcretes appear to be late Cretaceous to Neogene in age and are associated with now-uplifted land surfaces (Roberts, 2003). However, given the coincidence of some sea level lowstands (e.g., during Marine Isotope Stage 4; Fisher et al., 2010) with periods of peak silcrete use at coastal MSA sites (e.g., Pinnacle Point; Brown et al., 2009, 2012), the possibility of submerged raw material sources should not be overlooked. Acknowledgments Funding for fieldwork and geochemical analyses was provided by the University of Brighton. DJN would like to thank Joanna Bullard for assistance in the field during the collection of silcrete samples. Peter Lyons assisted with sample preparation. We also extend our thanks to the two anonymous reviewers whose helpful insights greatly improved the manuscript. Geochemical analyses were undertaken by ALS Minerals, Sevilla, Spain, and are certified under quality control certificate SV12045103 issued on 26 March 2012. Appendix. Supplementary data Supplementary methodological information (Appendix A) and data (Appendix B) related to this article can be found at http://dx. doi.org/10.1016/j.jhevol.2013.07.006. References Ambrose, S.H., 2006. Howiesons Poort lithic raw material procurement patterns and the evolution of modern human behavior: a response to Minichillo (2006). J. Hum. Evol. 50, 365e369. Ambrose, S.H., 2012. Obsidian dating and source exploitation studies in Africa: implications for the evolution of human behavior. In: Liritzis, I., Stevenson, C. (Eds.), Obsidian and Ancient Manufactured Glasses. University of New Mexico Press, Albuquerque, pp. 56e72. Avery, G., Cruz-Uribe, K., Goldberg, P., Grine, F.E., Klein, R.G., Lenardi, M.J., Marean, C.W., Rink, W.J., Schwarz, H.P., Thackeray, A.I., Wilson, M.L., 1997. The 1992e1993 excavations at the Die Kelders Middle and Later Stone Age Cave Site, South Africa. J. Field Archaeol. 24, 263e291. Avery, G., Halkett, D., Orton, J., Steele, T., Tusenius, M., Klein, R., 2008. The Ysterfontein 1 Middle Stone Age rock shelter and the evolution of coastal foraging. Goodwin Ser. Curr. Themes Middle Stone Age Res. 10, 66e89. Bosazza, V.L., 1939. The Silcrete and Clays of the Riversdale-Mossel Bay Area, vol. 32. Fulcrum, Johannesburg, pp. 17e29. Brown, K.S., Marean, C.W., Herries, A.I.R., Jacobs, Z., Tribolo, C., Braun, D., Roberts, D.L., Meyer, M.A., Bernatchez, J., 2009. Fire as an engineering tool of early modern humans. Science 325, 859e862. Brown, K.S., Marean, C.W., Jacobs, Z., Schoville, B.J., Oestmo, S., Fisher, E.C., Bernatchez, J., Karkanas, P., Matthews, T., 2012. An early and enduring advanced technology originating 71,000 years ago in South Africa. Nature 491, 590e594. Butzer, K.W., 1978. Sediment stratigraphy of the Middle Stone Age sequence at Klasies River Mouth, Tsitsikama Coast, South Africa. S. Afr. Archaeol. Bull. 33, 141e151. Charrié-Duhaut, A., Porraz, G., Cartwright, C.R., Igreja, M., Connan, J., Poggenpoel, C., Texier, P.-J., 2013. First molecular identification of a hafting adhesive in the Late Howiesons Poort at Diepkloof Rock Shelter (Western Cape, South Africa). J. Archaeol. Sci. 40, 3506e3518. Deacon, H.J., 1989. Late Pleistocene palaeoecology and archaeology in the Southern Cape, South Africa. In: Mellars, P., Stringer, C. (Eds.), The Human Revolution:
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